Gion Matsuri is Kyoto's biggest festival. It began in 869 as a kind of purification ritual to appease the gods in response to a plague that was ravaging Kyoto, and was made an annual event in 970. It consists of many events spread throughout the month of July, but the climax is undoubtedly the procession of thirty-two ornate and impressive floats through the streets of the main business district. The floats are decorated with valuable tapestries and brocades, and the largest carry traditional artists and musicians. Smaller floats carry models of famous historical characters. The procession takes place on July 17.
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Find out more about Yasaka Shrine.
You may not be aware but Gion Festival also has several events besides the big procession. I recommend Yoyama and Yoyoyama - the two nights before the procession. You can walk around and look at the hokko parked on narrow side streets. You can even go right onto one of the hokko for a small charge. The distinctive Gion music is everywhere. Some machiya owners open up their homes to visitors to see screens and other interesting antiques. Also lots of yummy festival food :-)
https://en.japantravel.com/kyoto/setting-up-for-gion-matsuri/13810
https://en.japantravel.com/kyoto/gion-matsuri-nights/13812